PROJECT SUMMARY Our long-term goal is to develop safe and effective drugs to treat obesity. The proposed studies are aimed at validating a new target, ALDH1A1, the major enzyme involved in generating retinoic acid (RA) in adipose tissues, for the treatment of obesity. RA is synthesized by three main aldehyde dehydrogenases, ALDH1A1, ALDH1A2, and ALDH1A3 that are expressed in temporally and spatially distinctive patterns and are involved in processes including development, reproduction and immunity. RA also regulates expression of genes involved in adipose tissue differentiation as well as glucose and lipid metabolism. During our efforts to develop male contraceptives using small molecules that reduce RA synthesis in the testes, we observed that mice treated with WIN 18,446, a pan-inhibitor of ALDH1A isozymes, had lower body weight gain due to a decrease in adipose tissue mass. This attenuated weight gain was reversed by co-treatment with RA, suggesting that ALDH1A inhibition (RA reduction) could be used for weight regulation. Thus, we tested the efficacy of RA synthesis inhibition using a murine model of diet-induced obesity and found that WIN 18,446 is effective in attenuating weight gain despite the animals? continued consumption of a high fat diet. Others have demonstrated that the absence of the RA synthesizing enzyme, Aldh1a1, alters energy metabolism in liver and adipose tissues due to changes in tissue retinoid levels (RA and retinal) and thus, protecting mice against diet- induced obesity. We hypothesize that pharmacological inhibition of RA biosynthesis by inactivating ALDH1A1 is a novel target for the treatment of obesity. To test this, in Specific Aim 1, we will evaluate efficacy and toxicity of WIN 18,446 as an obesity treatment and will additionally determine potential mechanisms, including alterations in energy metabolism in liver, adipose tissues and muscles, increased thermogenesis in adipose tissue and effects on adipogenesis. Retinoid concentrations and the expression pattern of the three RA synthesis enzymes will also be examined in human visceral and subcutaneous adipose tissues to determine whether differences in RA homeostasis (retinal and RA concentrations) correlate with body mass index in humans. Although WIN 18,446 is a promising compound for weight regulation, its inhibition of all three RA synthesis enzymes and other ALDH enzymes has been shown to cause unwanted side effects such as inhibition of spermatogenesis and alcohol toxicity. In Specific Aim 2, we propose to develop ALDH1A1 specific inhibitors to treat obesity, minimizing the potential side effects associated with complete inhibition of retinoic acid synthesis. In Specific Aim 3, we will test the efficacy of our new ALDH1A1- specific inhibitors for the treatment of obesity in mice. The proposed studies will identify a novel target to treat obesity with the potential to improve metabolic abnormalities associated with obesity.